ELECTRO-MAGNETISM. 123 



able ore from the refuse earth with which it is mixed. 

 A steel bar introduced into the helix while the current 

 is passing, becomes permanently magnetic. Permanent 

 magnets, are now commonly made in this manner. 



293. PERMANENT MAGNETISM OF STEEL. 



What effect 



has the mag- It appears, from the last paragraph, that 



"ted I 11 1 * a bar f S ft ir011 1S a ma g net > as lon g a S 



an electrical current circulates around it. 

 But the steel, if once magnetic, remains so permanent- 

 ly. This is accounted for, by supposing that the cur- 

 rent, in the wire, excites a current in the surface of the 

 steel itself, which continues to flow, without interrup- 

 tion, after the wire is removed. 



294. ACTION OF A SINGLE WIRE ON 



What is the 



action of a A MAGNET. ^-A wire, carrying a cur- 

 t magnet? * rent in the direction shown in the 

 figure, acts on a magnet, precisely as 

 on a floating coil. The north pole of the mag- 

 net is made to deviate to the east. The mo- 

 tion is such as to bring adjacent currents in wire 

 and magnet to coincide. 



295. ELECTRICAL THEORY OF MAG- 



Explain the . 



electrical theo- NETisM. According to this theory, all mag- 

 T tlm? magne ' netism ? including that of the load-stone, 

 the magnetic needle, and that of the earth 

 itself, is a consequence of the circulation of electrical 

 currents. In the earth, such currents are known to be 

 excited, and kept in motion, by the sun, heating in turn 

 successive portions of its surface. They flow from 

 east to west, making of the earth, as it were, an im- 

 mense coil, or helix. In magnets they are also in con- 



